1. Field of the Invention
The present invention relates to a method, apparatus, and system for inspecting the surface or interior of transparent objects, such as quartz substrates for semiconductor devices, photomasks formed of quartz, and pellicle membranes used as dust protectors in the manufacture of semiconductor devices, such as LSIs an VLSIS, or liquid crystal display panels.
2. Description of the Related Art
In the manufacture of semiconductor devices, such as LSIs and VLSIs, or liquid crystal display panels, a semiconductor wafer or a liquid crystal material panel is irradiated, via a photomask, with light to thereby form circuit patterns on the wafer or liquid crystal material panel. If dust adheres to the photomask, the dust absorbs or deflects light, causing a transferred pattern to deform, a pattern edge to become rugged, or a white background to be blackishly smudged, and thus impairing dimensional accuracy, quality, or appearance. As a result, the manufacture of semiconductor devices or liquid crystal panels has involved a problem of impairment in the performance of manufactured semiconductor devices or liquid crystal panels or a decrease in manufacturing yield. Thus, the irradiation of a photomask with light is usually performed in a clean room. However, even in a clean room, keeping a photomask completely clean is difficult. Hence, there is employed a method of bonding a pellicle membrane having good transmission of exposure light onto the surface of a photomask for the purpose of protecting the mask surface from dust.
Through use of such a pellicle membrane, dust, if any, does not directly adhere to the surface of a photomask, but adheres to the pellicle membrane. Thus, by focusing light on the surface of the photomask where a circuit pattern exists, the dust adhering on the pellicle membrane has no effect on the transfer.
A transparent pellicle membrane for the above application is formed from a material having good transmission of visible light, such as nitrocellulose, cellulose acetate, modified polyvinyl alcohol, or a fluorine polymer, and is bonded onto a frame of aluminum alloy, stainless steel, polyethylene, or a like material. In bonding, a good solvent for the pellicle membrane is applied onto the frame, and then the pellicle membrane is placed on the frame, followed by air drying. Alternatively, the pellicle membrane is bonded onto the frame through use of adhesive such as an acrylic resin or epoxy resin. Further, on the opposite side of the frame are formed an adhesive layer formed from a polybutene resin, polyvinyl acetate resin, acrylic resin, or the like for adhesion to a photomask and a release layer for protection of the adhesive layer.
In view of the above-mentioned application of a pellicle membrane, both side surfaces and interior of the pellicle membrane must be free of foreign matter. Accordingly, the pellicle membrane undergoes strict inspection.
Also, quartz substrates for semiconductor devices and quartz photomasks bearing circuit patterns formed thereon must have transparency and be free of defects to a degree equal to or higher than in the case of pellicle membranes. Thus, the surfaces or interior of such transparent objects are strictly inspected for foreign matter.
Among conventional inspection methods for inspecting the surfaces or interior of transparent objects, a conventional inspection method for pellicle membranes is exemplified in FIG. 5. As shown in FIG. 5, a frame to which a pellicle membrane A is bonded is attached to a handling jig. In a darkroom for inspection, an inspector holds the jig and exposes the surface of the pellicle membrane A to a spot-light from a convergence lamp B, to thereby visually check for foreign matter adhering to the membrane, foreign matter or defects present inside the membrane, and wrinkles or scratches in the membrane.
In the conventional method, the spot-light from a convergence lamp reflected from the surface of the pellicle membrane or from the pellicle frame impinges on and dazzles the inspector""s eyes, thus imposing a burden on the eyes. Accordingly, an adverse effect is imposed on the inspector""s eyes, and the inspector cannot continue the inspection for a long period of time. Also, inspection efficiency is impaired, and the inspector is apt to overlook foreign matter, resulting in a possible impairment in detection accuracy. These unfavorable phenomena are observed not only in the inspection of pellicle membranes but also in the inspection of quartz substrates and photomasks. Thus, there has been eager demand for a solution to the problems.
Also, conventional inspection methods for inspecting the surfaces or interior of transparent objects other than the above-mentioned visual inspection method include a method in which a transparent object is irradiated with a laser beam, and light scattered by foreign matter present on the surface or in the interior of the object is detected by a photomultiplier (a first method), and a method in which foreign matter itself is magnified and detected through use of a charge coupled device camera (hereinafter referred to as a CCD camera) equipped with a microscope (a second method).
Further, there has been proposed an inspection method for inspecting a transparent object for defects through use of a CCD camera (Japanese Patent Application Laid-Open (kokai) No. 4-344447) (a third method). In this method, a transparent object is irradiated with light in three directions, and light scattered by a defect such as scratch or smudge is detected by a CCD camera.
The first method has an advantage of very high repeatability of measurement, but has the following disadvantage. For example, for the portion (several millimeters wide) of a pellicle membrane in the vicinity of or along a pellicle frame, measurement is disabled due to interference of the frame with a laser beam, scatter of the skirt portion of intensity distribution (usually Gaussian distribution) of a laser beam caused by the frame, the diffraction effect of a laser beam, or the like.
The second method is not practicable for the following reason. In order to detect foreign matter having a small grain size, the magnifying power of the microscope must be increased, and thus inspection takes a very long time.
The third method is suited for inspecting a transparent object for two-dimensional defects such as scratches and smudges, but cannot properly inspect a transparent object for three-dimensional defects, for example, foreign matter, because three-dimensional defects are significantly different from two-dimensional defects in terms of scattering of light. Further, in the case of inspecting a pellicle membrane, since the pellicle membrane has a frame attached at the periphery thereof, the portion of the pellicle membrane along the frame cannot be inspected by methods, such as the above-described third method, in which a transparent object is irradiated with light in three directions concurrently. Thus, this method is not suited for inspection of pellicles.
Accordingly, inspection apparatuses which carry out the methods described above involve the same problems.
The present invention has been accomplished in view of the problems, and an object of the invention is to provide an inspection method for transparent objects that enables easy and efficient visual inspection of a transparent object with a reduced burden on the inspector""s eyes, as well as accurate judgment.
Another object of the present invention is to provide an inspection apparatus and inspection system which do not involve visual inspection and employ reflected light, and which is free from the above problems involved in conventional inspection apparatuses.
According to a first aspect of the present invention, there is provided an inspection method for a transparent object comprising the steps of irradiating a transparent object with light and inspecting the surface or interior of the transparent object, wherein transmitted light is observed on the side of the transparent object that is opposite to a light source.
Through observation of transmitted light having passed through a transparent object, the eyes of an inspector are not dazzled, in contrast with the case of observing reflected light from the surface of a transparent object; thus, the eyes are not burdened with light, the inspector does not overlook defects, and excellent detection accuracy and improved inspection efficiency are provided.
Preferably, the observation for inspection is performed visually. Also, preferably, the inspection is performed to detect foreign matter present in the surface or interior of the transparent object.
When a transparent object is to be visually inspected for foreign matter present on the surface or in the interior of the transparent object, the method of the invention is particularly effective because the eyes of an inspector are not dazzled by light. Further, foreign matter can be readily detected more accurately than in the case of mechanical detection, for example, detection through use of an Hexe2x80x94Ne-laser-type foreign matter inspection machine.
Further, preferably, the transparent object to be inspected is a pellicle membrane.
Through application to the inspection of a pellicle membrane, which must have particularly high transparency and must not carry even foreign matter as small as that on the submicron order, the inspection method of the invention exhibits its effectiveness.
According to a second aspect of the present invention, there is provided an inspection apparatus for a transparent object, comprising: transparent-object-moving means for moving a transparent object to an inspection position and for fixing the transparent object in the inspection position; irradiation means for emitting light from a light source disposed on one side of the transparent object so as to irradiate light onto the transparent object fixed in the inspection position by the transparent-object-moving means; and detection means located on the side of the transparent object opposite the light source and having a detector for detecting light that has been emitted from the light source and has passed through the transparent object.
The inspection apparatus enables an inspector to inspect a transparent object for foreign matter in a nonvisual manner and to accurately inspect a number of transparent objects in a short period of time.
Preferably, the detector is a CCD camera.
When, for example, a pellicle is to be inspected for foreign matter, the employment of a CCD camera as the detector enables the inspection of the portion of a pellicle membrane in the vicinity of and along a pellicle frame as in the case of visual inspection.
Preferably, the CCD camera as the detector is a digital CCD camera having a quantum efficiency of not less than 10%, a full well capacity of not less than 30,000 electrons, and a gradation number of not less than 256.
The above property ranges are specified for the CCD camera, because when the quantum efficiency indicative of sensitivity, the full well capacity indicative of dynamic range, and the gradation number indicative of resolution for quantity of light are in the above-described ranges, the digital CCD camera exhibits balanced performance and good detection efficiency when used as the detector.
Examples of such a digital CCD camera include digital cooled CCD cameras and digital CCD line-sensor cameras.
Further, preferably, the detector is positioned such that it is not directly irradiated with a bundle of rays from the light source of the irradiation means.
Through such positioning of the detector, a CCD camera, when used as the detector, does not suffer saturation of electric charge, and thus an inspection for foreign matter can be performed accurately.
Needless to say, the position of irradiating a transparent object with a bundle of rays from the light source of the irradiation means, i.e. a portion to be inspected, must be located within the detection range of the detector. Otherwise, no foreign matter can be detected; in other words, the inspection apparatus fails to achieve the expected function.
Preferably, the detector detects transmitted light which has passed through a transparent object to be inspected and has been scattered by foreign matter present on the surface or in the interior of the transparent object.
In the inspection apparatus of the present invention, foreign matter present on the surface or in the interior of a transparent object is irradiated with light which is emitted from the light source and passes through the transparent object, and light scattered by the foreign matter is detected by the detector.
Preferably, the transparent object to be inspected by the inspection apparatus of the present invention is a pellicle membrane.
Further, preferably, the light source is a high-intensity halogen lamp having a luminance of not less than 5,000 lux.
Through use of a high-intensity halogen lamp as the light source, a transparent object can be irradiated with light over a relatively wide area, and relatively high uniform illuminance can be obtained. Thus, the transparent object can be inspected more efficiently than in the case of inspection through use of laser beam.
According to a third aspect of the present invention, there is provided an inspection system for a transparent object, comprising: (a) an inspection apparatus section comprising an inspection apparatus according to the second aspect; (b) a control section for controlling the transparent-object-moving means, irradiation means, and detection means of the inspection apparatus section; (c) an image processing section for image-processing light detected by the detection means; (d) an analyzing section for providing the control section with information about control of the positions of the transparent-object-moving means, the irradiation means, and the detection means and for analyzing the result of processing conducted in the image processing section; and (e) an inspection result display section for displaying the result of analysis conducted in the analyzing section.
Through employment of the inspection system, light detected by the detector of the inspection apparatus section undergoes image-processing in the image processing section, and the result of the image processing is analyzed in the analyzing section, so that the result of the analysis, i.e. the size, amount, and position of foreign matter located in a transparent object, can be displayed on the inspection result display section in the form of a map. From the data displayed on the inspection result display section, an inspector can readily judge whether the inspected transparent object is usable, whether foreign matter must be removed, or from what position foreign matter must be removed.
Also, for reasons similar to those mentioned in relation to the inspection apparatus of the present invention, the detector of the inspection apparatus section is preferably a CCD camera. More preferably, the CCD camera used as the detector is a digital CCD camera. Particularly preferably, the digital CCD camera has a quantum efficiency of not less than 10%, a full well capacity of not less than 30,000 electrons, and a gradation number of not less than 256.
Also, preferably, the detector of the inspection apparatus section is positioned such that it is not directly irradiated with a bundle of rays from the light source of the inspection apparatus section.
Through such positioning of the detector, the detection accuracy of the detection means is improved. Also, in this case, the position where a transparent object is irradiated with light from the light source must be located within the detection range of the detection means.
According to the inspection method for transparent objects of the present invention, an inspector visually observes transmitted light which has passed through a transparent object on the side opposite to the light source with respect to the pellicle; thus, the inspector""s eyes are not dazzled by light. Accordingly, the inspector can readily find fine foreign matter, if any, so that highly accurate inspection can be carried out in a reduced period of time. Further, the burden on the inspector""s eyes is reduced, and inspection efficiency is significantly improved.
Further, according to the inspection apparatus and inspection system for transparent objects of the present invention, a transparent object can be inspected for foreign matter present on the surface or in the interior of the transparent object without use of reflected light. Accordingly, a number of transparent objects can be inspected without foreign matter being overlooked. Thus, the present invention enables a user to conduct highly accurate inspection of transparent objects.